The present invention relates generally to a Radio Frequency (RF) modem and more particularly to a spread spectrum RF modem that utilizes SAW devices for the resonator during transmission and the correlator during receiving and transmitting, both SAW devices fabricated on the same monolithic substrate.
As the use of computers continues to increase at a rapid rate, the demand for peripherals and systems connected via wireless connections continues to increase. The number of wireless of applications is currently increasing at a very high rate in areas such as security alarms, networking, data communications, telephony and computer security.
Wireless communications currently may take many forms such as ultrasonic, IR and RF. A commonly used communication technique in RF wireless communications is spread spectrum. Spread spectrum communication is a communication technique whereby the transmitted signal is spread over a frequency band that is significantly wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems have reduced susceptibility to interference and jamming thus enabling high levels of data integrity and security. Further, since the signal spreading process spreads the transmission power over a wide bandwidth, the power levels at any given frequency within the bandwidth are reduced significantly thereby reducing interference to other radio devices.
Spread spectrum communication systems are generally of the direct sequence (DS) type, the frequency hopping (FH) type or are a hybrid of the two that combine DS and FH. In direct sequence spread spectrum communications, a data signal is modulated with a pseudo random chip code so as to generate a transmitted signal whose frequency spectrum is spread over a wide bandwidth. The transmitted signal has a low spectral density and appears as noise to receivers lacking the code sequence. Thus, spread spectrum communications provides increased security for the data transmitted and reduced interference with other transmitters and receivers operating in the same environment.
The role of the transmitter in a spread spectrum communications system is to spread the signal in accordance with the data to be transmitted. Each bit or set of bits to be transmitted is converted into a plurality of chips having a much wider bandwidth than the original data. The spreading is performed in accordance with the code sequence chosen for the system.
The role of the receiver is to despread the spread spectrum signal in order to recover the original data signal. In direct sequence spread spectrum, the despreading of the signal is accomplished by correlating the received signal with a reference code matching the pseudo noise code used by the transmitter to transmit the information. As a consequence of de-spreading the signal, any interfering signals are also spread. The interfering signals typically comprise pseudo-random noise rather than cyclic noise that is easier to combat.
One technique for spread spectrum correlation is to convert the received signal into digital form before inputting it to a digital matched filter. Other spread spectrum correlation techniques utilize surface acoustic wave (SAW) devices to perform correlation on a received spread spectrum signal. SAW devices are thin film planar devices that permit propagation of acoustical waves on the free surface. The SAW device functions to convert electrical signals into acoustical signals and back again via piezo electric transducers.
SAW devices are useful in a variety of applications including spread spectrum correlators since they are generally capable of operating over a wide bandwidth. A SAW correlator device is a passive component constructed to recognize a specific sequence of code chips (similar in operation to a digital matched filter correlator) via correlation of phase shifts in an RF signal. The SAW correlator functions analogously to a delay line matched filter. It consists of many delay elements each having a delay period equal to the period of the transmitted code clock such that, at any time, each element corresponds to a single chip of the received signal.
As the received signal propagates down the SAW device, the phase structure of each element is added in or out of phase with the propagated wave. The outputs of all the elements may be summed to reach a maximum at a total correlation value. When the phase shift structure of all the elements matches the phase shifts of the propagated wave, a maximum sum, i.e., correlation, is achieved.
Since SAW devices are by nature fixed devices, a SAW correlator is usually programmed at the time of manufacture to match a single predetermined chip code sequence. The phase shift structure of the SAW device is programmed at the time of construction through transducers placed in each element to produce an elemental phase match and cannot be changed once manufactured thereby permitting correlation with a single code sequence.
It would therefore be desirable to have a RF modem that utilizes direct sequence spread spectrum techniques that can be constructed at low cost and small size. It is also desirable that such a RF modem utilize SAW devices for both the transmitter resonator, correlator and receiver correlator thereby reducing the size and cost of the modem.
The present invention is a bidirectional direct sequence spread spectrum half-duplex RF modem. The RF modem can be applied to transmit and receive numerous types of analog and digital pulse modulation. While the RF modem can be adapted to operate in numerous frequency ranges, an example is presented herein that is constructed to operate in the 902 to 928 Industrial, Scientific and Medical (ISM) band of frequencies. In addition, examples are provided that utilize the RF modem of the present invention to construct various types of data communications systems.
A key feature of the present invention is the incorporation in the RF modem of two different Surface Acoustic Wave (SAW) devices fabricated on a single monolithic substrate. A first SAW device is used as the resonator in the oscillator portion of the transmitter while a second SAW device forms the correlator for use in both the transmit and receiver portions of the modem. Another key feature of the invention is the very low amount of power consumed by the modem.
The RF modem is constructed to operate as a pulse transmitter and receiver. It is adapted to be generic in the sense that it is versatile enough to be used in many different types of data communication systems, several examples of which are presented below. The RF modem can be used as the physical (PHY) layer in a layered communication system such as the ISO OSI communication stack. As an example, the pulse transmitter RF modem, can be used to provide various modulation schemes including, but not limited to On/Off Keying (OOK), Pulse Width Modulation (PWM), Pulse Position Modulation (PPM) or any other type of analog or digital pulse modulation.
The transmit portion of the modem comprises an oscillator that uses a SAW resonator device. The output of the oscillator is switched on and off in accordance with the data to be transmitted. The pulse is input to the SAW correlator that functions to output the spreading waveform comprising a code sequence. Effectively, the SAW correlator functions as a BPSK modulator. The code sequence used is a 13-bit Barker code that is adapted to have high autocorrelation properties. The spreading sequence is amplified and transmitted via an antenna.
At the receiver, the received signal is first filtered by a band pass filter before being amplified by a Low Noise Amplifier (LNA). The amplified signal is input to the matched filter/correlator where a match with the Barker code sequence is detected. If a match is detected, a de-spreading pulse is output representing the original pulse. The output of the correlator is input to a peak detector that functions to detect, in a either a linear or non-linear fashion the envelope of the received signal. A dynamic reference signal is generated and -used to bias the threshold used to generate the binary output data signal.
The output power PT of the RF modem of the present invention is approximately 10 dBm. The processing gain is approximately 11 dB. In accordance with the FCC, providing a processing gain of at least 10 dB using direct sequence spread spectrum techniques permits the use of the higher output power level of 30 dBmi. Together, the effective output power PTEFF is on the order of 20 dBm. This translates to a maximum distance of communication (depending on actual conditions) of approximately 1000 meters. The maximum pulse rate achievable with the example RF modem presented herein constructed in accordance with the present invention is approximately 1.5 Mpps.
In an alternative embodiment, the transmission bit rate is increased by using a plurality of correlators wherein each is configured with a unique function (i.e., code) that is orthogonal with all other functions, i.e., they have near zero cross correlations with each other. The host is adapted to provide N data input and output lines. Each correlator having its own data input and output signal lines. The oscillator signal is generated by an oscillator circuit common to all correlators. An RF power splitter/combiner functions to combine the N transmission signals into a combined transmission signal and to split the received combined signal into multiple receive signals that are then fed to each correlator.
The RF modem of the present invention has a benefit of being relatively inexpensive to implement for the following reasons: (1) the size of both the required silicon and the SAW resonator and correlator devices are relatively small resulting in inexpensive manufacturing and high yield; (2) the high yield, as well as the simplicity of the devices, results in relatively simple testing of the components; and (3) the size of the resulting dies enables standard, inexpensive packaging.
The use of direct sequence spread spectrum technique provides numerous advantages, including the following: (1) the modem is adapted to transmit and receive very narrow pulses which is very desirable for pulse transceiving; (2) 10 dB is added to the effective transmission power per pulse due to the spread spectrum processing gain; (3) inherent immunity to interference; (4) inherent filtering of out of band noise; (5) inherent spreading of in-band noise; (6) a higher dynamic range available for communication; and (7) power savings resulting from fast oscillator wake-up time.
There is provided in accordance with the present invention a direct sequence spread spectrum radio frequency (RF) modem comprising an oscillator having a resonator and adapted to generate an oscillator signal, the center frequency of the oscillator determined by the center frequency of the resonator, switching means for gating the oscillator signal in accordance with input data to be transmitted so as to generate a series of pulses, spreading means for spreading the pulse output of the switching means with a spreading code sequence waveform so as to generate a spread spectrum transmission signal, means for transmitting the spread spectrum transmission signal, means for receiving the spread spectrum transmission signal, correlator means adapted to de-spread the spread spectrum transmission signal in accordance with the code sequence so as to generate a correlator signal, detection means for detecting the envelope of the correlator signal so as to generate a detection signal and decision means for applying a threshold to the detection signal so as to generate an output data signal therefrom.
The resonator comprises a surface acoustic wave (SAW) resonator. The switching means comprises a plurality of switches coupled in series and operative to provide a high level of electrical isolation. The plurality of switches may comprise a plurality of Field Effect Transistor (FET) switches. The spreading means comprises a surface acoustic wave (SAW) matched filter/correlator. The spreading code sequence comprises a Barker code series sequence, e.g., the 13-chip Barker sequence {1, 1, 1, 1, 1, xe2x88x921, xe2x88x921, 1, 1, xe2x88x921, 1, xe2x88x921, 1}.
The means for transmitting comprises an output amplifier for amplifying the spread spectrum transmission signal and an antenna coupled to the output of the output amplifier. The means for receiving comprises an antenna adapted to receive RF signals, a band pass filter coupled to the antenna and a low noise amplifier coupled to the output of the band pass filter. The correlator means comprises a surface acoustic wave (SAW) matched filter/correlator. The SAW matched filter/correlator is configured with a Barker code series sequence, e.g., the 13-chip Barker sequence {1, 1, 1, 1, 1, xe2x88x921, xe2x88x921, 1, 1, xe2x88x921, 1, xe2x88x921, 1}.
The spreading means and the correlator means share a surface acoustic wave (SAW) correlator adapted to be used half-duplex for transmission and receiving. The resonator means comprises a surface acoustic wave (SAW) resonator while the spreading means and the correlator means share a surface acoustic wave (SAW) correlator adapted to be used half duplex for transmission and receiving and wherein the SAW resonator and the SAW correlator are constructed on the same monolithic substrate. The detection means comprises a slow peak detector adapted to generate a slowly varying reference signal and a fast peak detector adapted to track the envelope of the correlator signal and to generate the detection signal therefrom. The decision means is adapted to generate a binary output signal in accordance with the detection signal applied against a threshold. The detection means is adapted to generate the threshold dynamically in accordance with the correlator signal.
There is further provided in accordance with the present invention a method of modulating and demodulating direct sequence spread spectrums, the method comprising the steps of generating an oscillator signal utilizing a resonator wherein the frequency of oscillation is determined by the center frequency of the resonator, gating the oscillator signal in accordance with input data to be transmitted so as to generate a series of pulses, spreading the pulses utilizing a spreading code sequence waveform so as to generate a spread spectrum transmission signal, transmitting the spread spectrum transmission signal, receiving the spread spectrum transmission signal, de-spreading the spread spectrum transmission signal in accordance with the code sequence so as to generate a correlator signal, detecting the envelope of the correlator signal so as to generate a detection signal and applying a threshold to the detection signal so as to generate an output data signal therefrom.
There is also provided in accordance with the present invention a direct sequence spectrum radio frequency (RF) modem comprising an oscillator having a resonator and adapted to generate an oscillator signal, the center frequency of the oscillator determined by the center frequency of the resonator, a plurality of N transmit/receive circuits, each the transmit/receive circuit comprising switching means for gating the oscillator signal in accordance with input data to be transmitted so as to generate a series of pulses, spreading means for spreading the pulse output of the switching means with a spreading code sequence waveform so as to generate a spread spectrum transmission signal, correlator means adapted to de-spread a spread spectrum receive signal in accordance with the code sequence so as to generate a correlator signal, detection means for detecting the envelope of the correlator signal so as to generate a detection signal and decision means for applying a threshold to the detection signal so as to generate an output data signal therefrom, wherein the correlator in each transmit/receive circuit is configured with a unique function substantially orthogonal to functions in other correlators, means for combining and transmitting the N spread spectrum transmission signals generated by the N transmit/receive circuits as a combined spread spectrum transmission signal, means for receiving and splitting the combined spread spectrum transmission signal into N spread spectrum receive signals and wherein N is a positive integer.
The means for combining the N spread spectrum transmission signals comprises an RF power combiner splitter and the means for splitting the combined spread spectrum signal comprises an RF power combiner splitter.
There is still further provided in accordance with the present invention a method of modulating and demodulating N direct sequence spread spectrum signals, each direct sequence spread spectrum signal associated with one of N channels, the method comprising the steps of generating N oscillator signals utilizing a resonator wherein the frequency of oscillation is determined by the center frequency of the resonator, for each channel: gating each oscillator signal in accordance with input data for the Nth channel to be transmitted so as to generate a series of pulses, spreading the pulses utilizing a spreading code sequence waveform so as to generate a spread spectrum transmission signal, the spreading code sequence waveform for the Nth channel substantially orthogonal with the spreading code sequence waveforms of all other channels, de-spreading a spread spectrum transmission receive signal in accordance with the code sequence so as to generate a correlator signal, detecting the envelope of the correlator signal so as to generate a detection signal and applying a threshold to the detection signal so as to generate an output data signal therefrom, combining and transmitting the N spread spectrum transmission signals as a combined spread spectrum transmission signal, receiving and splitting the combined spread spectrum transmission signal into N spread spectrum receive signals and wherein N is a positive integer.